RFR: Idempotent builds

[Jonathan Gibbons]

In aid of idempotent builds ...

Replace `<delete>` before `<javac>` with `<depend>`
Remove `<delete>` before copying help files

For now the <delete> tasks are just commented out. If you agree, they can be removed.

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Commit messages:
 - Merge with upstream
 - avoid use of <delete> task

Changes: https://git.openjdk.java.net/jtharness/pull/6/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtharness&pr=6&range=00
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtharness/pull/6.diff
  Fetch: git fetch https://git.openjdk.java.net/jtharness pull/6/head:pull/6

PR: https://git.openjdk.java.net/jtharness/pull/6